With the advent of hypodermic needles and especially stainless steel needle stock, access was provided to the blood system via a vein for performing transfusions, adding plasma, biological saline solution, glucose and drugs to the body. The pressure required to drive the fluid into the vein, overcoming the venous pressure and the pressure drop of the indwelling needle or catheter, is provided by the hydrostatic head produced by raising the supply vessel above the patient. However, for various reasons, the rate of administering the fluid had to be controlled; for instance, when drugs were administered and it was desired to maintain the optimal concentration of drug in the blood system, and when the introduced fluid had a detrimental effect on the vein lining and a slower delivery rate was desired to provide quicker dilution of the drug. These factors gave rise for the need of flow control means
From the beginning of I.V. administration to the present time the predominant flow control means was to partially occlude the supply line by clamping means. Of these clamping means the most accepted was the Roller Clamp. However, this means has a common problem caused by the stress induced plastic yielding of the universally used plastic tubing of the administration sets. When the plastic tubing was partially occluded by clamping, the stresses set up in the tubing are slowly released by plastic flow. The result is that the flow rate changes with time, necessitating readjustment of the flow rate one or more times until an stable condition is reached. Nevertheless, Roller Clamps are presently the most used flow control means.
Other I.V. flow rate control means have come to market. One approach has been via an adjustable flow restrictor, wherein the effective length of the flow passage is changed by the adjustment means. Another approach is the use of a differential pressure regulator that provides a constant differential pressure that is applied to a separate, adjustable flow length restrictor. Adjustment of this flow restrictor varies the length of a variable cross-section channel the fluid must traverse. At one end of the adjustment range the channel is occluded for shut-off, at the other end of the adjustment range the channel is bypassed for full flow. This design appears to fulfill a need, but is limited by its manufacturing cost and a propensity to plug.
Both of the above two approaches suffer from the fact that the flow channel must be sealed on both sides for their entire length to prevent leakage externally or along the sides of the channel thereby decreasing the impedance of the flow restrictor. A tight fit of the mating parts to prevent leakage results in too high a torque for acceptable ease of adjustment and conversely, if the adjustment torque is reduced by easing the interference fit, leakage occurs. The worse case occurs if the controller is mounted close to the supply reservoir, in which case the external pressure is greater than the pressure in the flow controller, resulting in air leakage into the I.V. solution. The window between acceptable adjustment torque and the threshold of leakage is small, resulting in difficultly of manufacturing and increased cost.
The present invention uses an internal, fixed orifice providing a hermetically sealed unit, except for the adjusting stem which is sealed by a silicone coated O-ring that provides a vacuum tight seal.